Assays for digoxin that have been commercially marketed all involve immunoassay technology. These immunoassays are used routinely in hospital clinical laboratories to monitor the serum digoxin concentration of patients taking digoxin. The strength of these immunoassays is their impressive sensitivity (about 0.1 ng/ml lower limit; the "therapeutic range" in patients is 0.5-2.0 ng/ml serum) and their ease of performance in a clinical laboratory. The important limitation is that the immonoassay does not measure only digoxin; it also measures interfering active and inactive metabolites of this drug, as well as endogenous interfering substances currently referred to as digitalis-like immunoreactive substances (DLIS).
Some investigators have combined high performance liquid chromatography (HPLC) with immunoassay in order to separate metabolites by HPLC and then detect them by immunoassay. The limitation of this technique is that some metabolites are not detected by immunoassay techniques. Others have combined HPLC with radioactivity detection. the limitation of the latter technique is that radioactive drug must be administered and therefore the method is not applicable in the normal setting of clinical use.
Digoxin and digitoxin are the two digitalis cardiac glycosides that are used most frequently in a clinical setting. Digoxin is used much more frequently than digitoxin and has been the subject of extensive scientific investigation, including investigations aimed at developing analytical methodology. For convenience, the assay techniques will be outlined.